Paper and paper board are produced by forming a a fiber mat from an aqueous cellulosic slurry and drying such fiber mat to provide a finished sheet which routinely has less than 6 weight percent of water. The fiber mat is formed on a moving wire (endless wire belt) or web, and is then subjected to dewatering and drying steps. The cellulosic slurry typically has a consistency (percent dry weight of solids in the slurry) of less than 1 percent, and commonly below 0.5 percent, at the time it is employed to form the wet fiber mat. Such low consistencies are generally necessary to produce a finished sheet having a reasonable formation. Such low consistencies routinely require that the cellulosic slurry be diluted ahead of the paper machine.
One aspect of papermaking that is extremely important to its efficiency and cost is the retention of furnish components on and within the fiber mat being formed during the papermaking process. A papermaking furnish may contain particles that range in size from about colloidal size, to the 2 to 3 millimeter size of cellulosic fibers. Within this range are cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles. Such small particles in the furnish would in significant portion pass through the spaces (pores) between the cellulosic fibers in the fiber mat being formed without the inclusion of one or more retention aids. Thus the inclusion of retention aids as wet end additives in the papermaking process is both widely practiced and very important to the process.
A greater retention of fines and fillers permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of less quality are employed to reduce papermaking costs, and reduce the demand on raw material supplies, the retention achieved becomes even more important because the fines content of lower quality pulps is greater than that of higher quality pulps.
A greater retention of fines, fillers and other slurry components reduces the amount of such substances that are lost to the white water, and hence reduces the amount of material waste, the cost of waste disposal and the adverse industrial and environmental effects of significant material loss to the white water.
Another important aspect of papermaking is the formation of the finished sheet. Formation is a measure of the uniformity of the paper sheet. Formation is generally determined by the variance in the light transmission property within a paper sheet, and a high variance is indicative of poor formation. When retention aids are utilized to increase retention, the formation property is generally seen to decline. The need for a reasonable formation is often a limiting factor in achieving higher levels of retention.
A further important aspect of the papermaking process is the efficiency of drainage of the wet fiber mat. As noted above, the cellulosic slurry is diluted to a consistency of less than one percent for the fiber mat formation stage, and the finished sheet has a water content of less than 6 weight percent. A significant amount of the water is removed while the fiber mat is on the wire. Initially the water may drain freely through the fiber mat and wire by gravitation force, and thereafter the consistency of the fiber mat on the wire may be raised to about 15 to 20 percent by the use of vacuum suction to remove water. After leaving the wire the fiber mat is dewatered further by means such as pressing, felt blanket blotting and pressing, evaporation and the like. In practice a combination of such methods are utilized to dry the sheet to the desired water content. Since free drainage is both the first and least expensive dewatering method used, its efficiency should at least be maintained in any papermaking process. The goals of increasing the retention while maintaining good formation should not be achieved at the expense of efficient drainage.
It is generally desirable to minimize the amount of additives employed for various purposes in a papermaking process, to the extent possible while obtaining the result sought. Additive minimization may realize material cost savings and handling and processing benefits. In addition, minimization of additives reduces the risks of adverse effects from such additives. For instance, the use of some wet end additives at high levels can be detrimental to other papermaking aspects, such as the dry strength of the finished paper sheet.
It is also generally desirable to use additives that may be delivered to the paper machine without undue problems. Additives that are easily dissolved or dispersed in water minimize the expense and energy required for delivering them to the paper machine and provide a more reliable uniformity of feed than additives which are not easily dissolved or dispersed.